Re: [PATCH 2/9] KVM: x86: simplify read_emulated

Xiao Guangrong
2012-07-23T04:23:22Z

On 07/21/2012 03:52 AM, Marcelo Tosatti wrote:
> On Fri, Jul 20, 2012 at 09:15:44PM +0800, Xiao Guangrong wrote:
>> On 07/20/2012 06:58 PM, Marcelo Tosatti wrote:
>>> On Fri, Jul 20, 2012 at 10:17:36AM +0800, Xiao Guangrong wrote:
>>>> On 07/20/2012 07:58 AM, Marcelo Tosatti wrote:
>>>>
>>>>>> - }
>>>>>> + rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
>>>>>> + &ctxt->exception);
>>>>>> + if (rc != X86EMUL_CONTINUE)
>>>>>> + return rc;
>>>>>> +
>>>>>> + mc->end += size;
>>>>>> +
>>>>>> +read_cached:
>>>>>> + memcpy(dest, mc->data + mc->pos, size);
>>>>>
>>>>> What prevents read_emulated(size > 8) call, with
>>>>> mc->pos == (mc->end - 8) now?
>>>>
>>>> Marcelo,
>>>>
>>>> The splitting has been done in emulator_read_write_onepage:
>>>>
>>>> while (bytes) {
>>>> unsigned now = min(bytes, 8U);
>>>>
>>>> frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
>>>> frag->gpa = gpa;
>>>> frag->data = val;
>>>> frag->len = now;
>>>> frag->write_readonly_mem = (ret == -EPERM);
>>>>
>>>> gpa += now;
>>>> val += now;
>>>> bytes -= now;
>>>> }
>>>>
>>>> So i think it is safe to remove the splitting in read_emulated.
>>>
>>> Yes, it is fine to remove it.
>>>
>>> But splitting in emulate.c prevented the case of _cache read_ with size
>>>> 8 beyond end of mc->data. Must handle that case in read_emulated.
>>>
>>> "What prevents read_emulated(size > 8) call, with mc->pos == (mc->end - 8) now?"
>>
>> You mean the mmio region is partly cached?
>>
>> I think it can not happen. Now, we pass the whole size to emulator_read_write_onepage(),
>> after it is finished, it saves the whole data into mc->data[], so, the cache-read
>> can always get the whole data from mc->data[].
>
> I mean that nothing prevents a caller from reading beyond the end of
> mc->data array (but then again this was the previous behavior).

1024 bytes should be enough for instructions, may be we can add a WARN_ON
to check buffer-overflow.

>
> ACK
>

Thank you, Marcelo!